We have a new, improved string type coming up in AK (OOM aware, no null
state), and while it's going to use UTF-8, the name UTF8String is a
mouthful - so let's free up the String name by renaming the existing
class.
Making the old one have an annoying name will hopefully also help with
quick adoption :^)
With so much infrastructure implemented, we can finally add the last
piece of this puzzle - the fetch() method itself!
This contains a few hundred lines of generated code as handling the
RequestInfo and RequestInfo parameter types manually is not feasible,
but we can't use the IDL definition as the Window object is handwritten
code at the moment.
It's neatly tucked away in Bindings/ and will be removed eventually.
